1. Field of the Invention
The present invention relates to improvements in a digital printing press equipped with an image-forming device.
2. Discussion of the Background
A digital offset printing press of the above type is illustrated in FIG. 5. The printing press includes a printing unit for printing on sheets, a sheet-feeding unit for feeding sheets onto the printing unit, and a sheet-discharging unit for discharging sheets printed at the printing unit to the outside of the printing unit.
The printing unit in turn includes impression cylinder 3, and two plate cylinders 1 and two blanket cylinders 2 respectively disposed on the upstream and downstream sides of the impression cylinder 3. The plate cylinders 1 each are provided on its outer circumference with a printing plate or printing plates. The blanket cylinders 2 each transfer ink images from the plate cylinder 1 onto its outer circumference. The impression cylinder 3 transfers the ink images on the blanket cylinders 2 onto oncoming sheets held on its outer circumference. The printing press having this arrangement is designed for printing on each sheet in two color during every one rotation of the impression cylinder 3. To achieve this two-color printing, the two plate cylinders 1 and two blanket cylinders 2 are arranged with a predetermined spacing from each other along the circumferential surface of the impression cylinder 3.
According to the above arrangement, the printing press can print on each sheet held on the impression cylinder 3 in a total of four colors during every time the impression cylinder 3 rotates twice. To achieve this operation, the plate cylinders 1 and the blanket cylinders 2 each are designed as a so-called double-diameter cylinder. That is, the plate cylinder 1 and the blanket cylinder 2 on each side respectively provide outer circumferences each being equally divided into two segments, on which ink images in different colors are respectively formed. The plate cylinder 1 on each side is provided on each segment with a different printing plate. The impression cylinder 3 is designed as a so-called three-diameter cylinder that provides the outer circumference equally divided into three segments, each of which segment receives thereon one sheet.
The printing press also includes image-forming devices 4 disposed on the upstream and downstream sides that are fixed in position with a predetermined spacing from the outer circumferences of the respective plate cylinders 1 for forming images on the printing plates mounted on the plate cylinder 1 on each side. The printing press is provided with two sets of ink-feeding units 5, each set comprised of two ink-feeding units 5 and disposed around the plate cylinder 1 on each side. A total of four ink-feeding units 5 are thus arranged in the printing unit.
The ink-feeding units 5 represented by triangles in FIG. 5 each include a number of ink rollers that respectively have rotational axes extending parallel with the axis of a corresponding cylinder and feed inks onto the printing plates through the rotations thereof. One or more of these ink rollers are designed as laterally-oscillating rollers, which are also referred merely to oscillating rollers. The oscillating rollers are reciprocally moved in the lateral direction or along the cylinder axes to feed inks onto the printing plates as even-outing the ink distribution along the lateral or width direction of the printing press (i.e. the direction orthogonal to the sheet feeding path).
On the other hand, the sheet-feeding unit disposed on the upstream side of the impression cylinder 3 (i.e., right-hand side of the impression cylinder in FIG. 5) includes a sheet-feeding cylinder 6 disposed as facing the impression cylinder 3. The sheet-feeding cylinder 6 transfers each sheet to an oncoming one of grippers 7 of the impression cylinder 3 every time the impression cylinder 3 rotates twice. The sheet-feeding cylinder 6 is designed as a single-diameter cylinder that receives a single sheet on its outer circumference.
The sheet-discharging unit disposed on the downstream side of the impression cylinder 3 (i.e., left-hand side of the impression cylinder 3 in FIG. 5) includes a pair of sheet-discharging sprockets 8, an endlessly-formed transfer chain 9 adapted to be driven through the pair of sheet-discharging sprockets 8, and sheet-discharging grippers 10 fixed on the transfer chain 9 as aligned along the lengthwise direction of the transfer chain 9 with a predetermined spacing from each other. The sheet-discharging grippers 10 each receive a printed sheet from the impression cylinder 3 every time the impression cylinder 3 rotates twice.
The thus arranged printing press is actuated by means of a single driving source. Specifically, the printing unit, the sheet-feeding unit, the sheet-discharging unit and the ink-feeding units 5 are actuated in synchronization with each other upon receiving the driving force of a single motor 11 mounted on the printing press. The driving force of the motor 11 is also received by roller-oscillating units which in turn convert rotary motion into linear motion via motion converting means to reciprocally move the oscillating rollers.
The driving-force transmission path from the motor 11 to the respective moving parts will be described below.
The driving force output from the motor 11 is transmitted to the plate cylinders 1 via the blanket cylinders 2. The driving force thus received by the plate cylinders 1 is transmitted to the respective ink rollers of the ink-feeding units 5 and the roller-oscillating units via different routes. The sheet-feeding unit and the sheet-discharging unit each receive the driving force of the motor 11 via the impression cylinder 3. Specifically, the driving force transmitted to the impression cylinder 3 is transmitted to both the blanket cylinders 2, and simultaneously to the sheet-feeding cylinder 6 of the sheet-feeding unit and the sheet-discharging sprockets 8 of the sheet-discharging unit for synchronous operation. Accordingly, all of these parts are actuated in synchronization with each other during the printing operation.
The printing press as mentioned above is a digital printing press that is capable of continuously printing on sheets of several lots without the necessity to replace the printing plates. That is, the printing press is designed to automatically form images on the printing plates mounted on the plate cylinders 1 by the image forming units 4. For the automatic image-forming operation, the plate cylinders 1 must be rotated. Specifically, the plate cylinders 1 with a plurality of segments must be rotated at high speed.
However, the rotation of the plate cylinders 1 by the motor 11 causes the simultaneous rotations of not only the blanket cylinders 2 and the impression cylinder 3, but also the ink rollers, the sheet-feeding cylinder 6 and the sheet-discharging sprockets 8, as well as causing the simultaneous reciprocal movements of the oscillating rollers. The sheet-feeding unit, the sheet-discharging unit, the ink-feeding units 5 and the roller-oscillating units do not directly contribute to the image-forming operation. Therefore, the simultaneous actuation of these units may cause not only an excessive load to the motor 11, but also a longer period of time for forming the images due to the lower speed operation of the plate cylinders 1. In addition, the printing press greatly vibrates during the image forming operation.
To address the above problems, the present applicant filed a patent application (Japanese Patent Application No. Hei 11-204793 and U.S. Pat. No. 6,302,018), in which the arrangement enabling shutting-off of the driving-force transmission from the motor 11 to the sheet-feeding unit and the other units during the image-forming operation by the image-forming devices was proposed.
Specifically, clutches are respectively disposed in the respective driving-force transmission paths from the motor 11 to the sheet-feeding unit, the sheet-discharging unit, the ink-feeding units 5 and the roller-oscillating units. More specifically, a total of six clutches are respectively disposed between the impression cylinder 3 and the sheet-feeding cylinder 6, between the impression cylinder 3 and the adjacent one of the sheet-discharging sprockets 8, between the plate cylinder 1 and a corresponding set of the ink rollers on each side, and between the plate cylinder 1 and the roller-oscillating units on each side, so that the driving-force transmission to the sheet-feeding units and the other units can entirely be shut off during the image forming operation. The shutting-off of the driving-force transmission to the units or parts irrespective to the image-forming operation achieves reduction of the load applied to the motor 11, higher speed operation of the plate cylinders 1, and hence shortening of the time period for the image-forming operation.
However, the above arrangement with the clutches disposed in each transmission path necessitates a large number of clutches, and hence may result in increased manufacturing cost of the printing press.
It is an object of the present invention to provide a low-cost digital printing press that is capable of shortening a time period for the image forming by reducing the load applied to the motor during the image forming operation.
According to one aspect of the present invention, there is provided a digital printing press with an image forming device fixedly mounted thereto for forming images on printing plates. The digital printing press includes a printing unit that in turn includes a plate cylinder having the printing plates mounted thereon for printing on sheets, a sheet-feeding unit for feeding sheets to the printing unit, a sheet-discharging unit for discharging sheets printed at the printing unit from the printing unit, an ink-feeding unit that includes a plurality of ink rollers that are rotated to respectively feed inks onto the printing plates mounted on the plate cylinder with some of the ink rollers being oscillating rollers that rotate as moving in the direction parallel to the axis of the plate cylinder, and a roller-oscillating unit for driving the oscillating rollers. The printing unit, the sheet-feeding unit, the sheet-discharging unit, the ink-feeding unit and the roller-oscillating unit are adapted to receive the driving force from a common motor. The image-forming device is adapted to form images on the printing plates as the plate cylinder rotates upon receiving the driving force from the common motor. The transmission of the driving force to at least two of the sheet-feeding unit, the sheet-discharging unit, the ink-feeding unit and the roller-oscillating unit is shut off by means of a single clutch during the image-forming operation.
With this digital printing press, it is possible to shorten the time required for forming images on the printing plates, and reduce the number of clutches as compared with a conventional arrangement that several clutches are disposed corresponding to each of the units.
According to another aspect of the present invention, there is provided a digital printing press with an image forming device fixedly mounted thereto for forming images on printing plates. The digital printing press includes a printing unit that in turn includes a plate cylinder having the printing plates mounted thereon for printing on sheets, a sheet-feeding unit for feeding sheets to the printing unit, a sheet-discharging unit for discharging sheets printed at the printing unit from the printing unit, an ink-feeding unit that includes a plurality of ink rollers that are rotated to respectively feed inks onto the printing plates mounted on the plate cylinder with some of the ink rollers being oscillating rollers that rotate as moving in the direction parallel to the axis of the plate cylinder, and a roller-oscillating unit for driving the oscillating rollers. The printing unit, the sheet-feeding unit, the sheet-discharging unit, the ink-feeding unit and the roller-oscillating unit are adapted to receive the driving force from a common motor. The image-forming device is adapted to form images on the printing plates as the plate cylinder rotates upon receiving the driving force from the common motor. The transmission of the driving force to all of the sheet-feeding unit, the sheet-discharging unit, the ink-feeding unit and the roller-oscillating unit is shut off by a clutching means during the image-forming operation. The transmission of the driving force to at least two units selected from the sheet-feeding unit, the sheet-discharging unit, the ink-feeding unit and the roller-oscillating unit is shut off by means of a single clutch.
Likewise, the digital printing press of this arrangement produce the same effects as described above.